Gianfranco Ercolani

Unraveling the Mechanism of the Soai Asymmetric Autocatalytic Reaction by First‐Principles Calculations: Induction and Amplification of Chirality by Self‐Assembly of Hexamolecular Complexes

case of amplifying asymmetric autocatalysis reported to date. Its behavior stands out as a paradigm for absolute asymmetric synthesis and the origin of homochirality in nature, yet the underlying mechanism remains elusive, thus prompting us to investigate the reaction by computational methods. The reaction is confined to the addition of [iPr2Zn] to aromatic aldehydes with at least one pyridinic nitrogen in the g-position, but the presence of a suitable substituent in the dposition, such as a methyl or a tert-butylethynyl group, is also important to reach the highest levels of chiral amplification. Blackmond and co-workers reported that the rate with the racemic catalyst is approximately half that with the enantiopure catalyst throughout the reaction, and ascribed this fact to the catalytic activity of homochiral dimers which are in statistical equilibrium with an inactive heterochiral dimer. They also found that the reaction rate is second-order in aldehyde, first-order in the active dimers, and independent of the concentration of [iPr2Zn]. [4c,d] The initially proposed structure for the dimers, 3, [4a] was successively refuted in favor of the structure 4, detected by NMR spectroscopy, although it should be noted that a catalytic intermediate is not necessarily the most abundant species. Since all the structural features noted above are essential for the success of the reaction, a meaningful computational study does require that all of them are retained in the calculations. Owing to the large size of the systems involved, energies of both ground and transition states (TSs) were calculated at the B3LYP/6-31G(d)//HF/3-21G(d) level of theory and corrected for ZPVEs. At the onset, wemade the reasonable working assumption that all the association processes occurring in solution are fast and reversible whereas the transfer of the isopropyl group from zinc to aldehyde is irreversible and rate limiting. The detected rate law indicates that in the TS there are two molecules each of 1 and 2, but the assumption above implies that at least one molecule of [iPr2Zn] must also be present. The high level of chiral induction detected suggests the fast self-assembly of an ordered complex in which the reactants are held in close proximity before the isopropyl transfer takes place. After several unsuccessful attempts to model a meaningful assembly made of two molecules of 1, two of 2, and one of [iPr2Zn], we passed to examine assemblies made of two molecules each of 1, 2, and [iPr2Zn]. At this level of complexity, we found an appealing homochiral structure, dubbed 7-(R,anti)2, with two symmetrically equivalent sides. For each side, the anti notation refers to the orientation of the isopropyl group bound to the R carbon relative to the neighbouring [iPr2Zn], as emphasized by the bond torsion in bold in the structure Anti. The structure provides a straightforward explanation not only for the chiral induction but also for the critical role of isopropyl groups and g-pyridinic nitrogen atoms. Indeed, the anti arrangement makes one of the zinc-bound isopropyl groups (that on the right in Anti) correctly oriented to attack the Re face of the aldehyde so as to reproduce the chirality of the R catalyst acting as template. The syn orientation, in the front side of the structure dubbed 7-R,anti-R,syn, which is predisposed for isopropyl attack at the “wrong” Si face, suffers from steric interactions among the isopropyl groups, as illustrated by the structure Syn. Scheme 1. Chiral amplification by the Soai autocatalytic reaction.

Linear Polynuclear Helicates as a Link between Discrete Supramolecular Complexes and Programmed Infinite Polymetallic Chains

The contribution of the solvation energies to the assembly of polynuclear helicates reduces the free energy of intermetallic repulsion, DeltaE(MM), in condensed phase to such an extent that stable D(3)-symmetrical tetranuclear lanthanide-containing triple-stranded helicates [Ln(4)(L4)(3)](12+) are quantitatively produced at millimolar concentrations, despite the twelve positive charge borne by these complexes. A detailed modelling of the formation constants using statistical factors, adapted to self-assembly processes involving intra- and intermolecular connections, provides a set of five microscopic parameters, which can be successfully used for rationalizing the stepwise generation of linear bi-, tri- and tetranuclear analogues. Photophysical studies of [Eu(4)(L4)(3)](12+) confirm the existence of two different binding sites producing differentiated metal-centred emission at low temperature, which transforms into single site luminescence at room temperature because of intramolecular energy funelling processes.

6-Azahemiporphycene: a new member of the porphyrinoid family.

The reaction of 5,10,15-triarylcorrole with 4-amino-4H-1,2,4-triazole provides another example of corrole ring expansion to give the corresponding 6-azahemiporphycene, a novel porphyrin analogue. The facile oxidation of the corrole ring is a required step for the ring expansion and for this reason the reaction fails in the case of corroles bearing meso-phenyl groups carrying electron-withdrawing substituents. Steric requirements also limited the scope of the reaction, which is not successful in the case of 2,6-disubstituted meso-aryl corroles. The occurrence of an initial oxidation is further supported by formation of the 6-azahemiporphycene derivative when the reaction is carried out under the same conditions, using a 5- or a 10-isocorrole as starting material. (1)H NMR spectra and X-ray crystal characterization of 6-azahemiporphycene evidenced the presence of an intramolecular N-H...N hydrogen bond in the inner core of the macrocycle, while photophysical characterization confirmed the aromatic character of the novel macrocycle, showing an intense Soret-like band around 410 nm in the absorption spectrum. The fluorescence emission is very modest, and 6-azahemiporphycene showed higher photostability than the corresponding corrole species. Different metal complexes of 6-azahemiporphycene were prepared following synthetic protocols usually exploited for the preparation of metalloporphyrins, demonstrating good coordination properties for the macrocycle. Both the free-base and metal derivatives were characterized by cyclic voltammetry and spectroelectrochemistry in dichloromethane and benzonitrile. To further detail the behavior of this novel macrocycle, density functional theory (DFT) calculations were carried out on the basic structure of 6-azahemiporphycene with the aim of assessing aromaticity and tautomerism, as well as calculating its stability with respect to the 5-aza isomer.

Ring-expanding polymerization by reversible ring fusion. a fascinating process driven by entropy

Grubbs et al. reported a successful method for synthesizing high-molecular weight cyclic polymers without the intermediacy of linear chains (Science 2002, 297, 2041). In spite of its practical significance, there are, however, important questions about this process that remain open. Here it is presented a theory of ring-expanding polymerization mediated by ring-ring equilibria, and driven by entropy only, that addresses the observed results. In particular the theory predicts the existence of a critical concentration (CC) below which only low-molecular weight cyclic oligomers are formed and above which the entire system is predicted to collapse into a single giant ring molecule, the process resembling an irreversible one. Dilution of the system below the CC is predicted to restore the mixture of low-molecular weight cyclic oligomers.